The thyroid gland is one that most people never give much thought to – until it causes problems. The gland itself is about two inches long and shaped like a butterfly. It sits in the front area of the neck just below the larynx with one lobe on each side of the windpipe. Along with the other components of the endocrine system, the thyroid’s job is to produce hormones. It can also store these hormones and release them into the bloodstream.

The hormones produced by the thyroid are very important, as they affect metabolism, weight, breathing, nervous system functions, muscle strength, body temperature, and menstrual cycles. If the thyroid produces too much or too little hormone, nearly every organ in the body can be affected. Hypothyroidism is underactivity of the thyroid; overactivity is called hyperthyroidism. Because the thyroid affects the menstrual cycle, a woman with a thyroid disorder may find it more difficult to get pregnant.

If a pregnant woman has a thyroid problem, there are special considerations to keep in mind. A woman with thyroid disease can certainly have a healthy, normal pregnancy and baby if she talks about this problem with her doctor, educates herself on the ways in which the thyroid is affected by pregnancy, stays up to date on her thyroid function testing, and takes the proper medications on the proper schedule.

How Is the Thyroid Affected by Pregnancy?

The thyroid gland is basically responsible for controlling our body’s metabolism. It is regulated by a hormone known as thyroid-stimulating hormone or TSH, which is secreted by the pituitary gland.Pregnancy causes the production of many hormones, one of which is human chorionic gonadotropin (hCG). In early pregnancy, the growing placenta makes human chorionic gonadotropin (hCG). HCG increases the production of progesterone, which is crucial for the growing fetus and without it may increase the risk for miscarriage. What does hCG and TSH have in common? Well, they are two different hormones; one (hCG) plays an integral role in maintaining pregnancy while the other (TSH) regulates the thyroid gland. However, hCG can mimic TSH , causing the thyroid gland to become hyperstimulated. It turns out that the molecular structure of both of these “different” hormones are very similar.

Both are composed of two different protein subunits. One of those protein subunits is called “alpha” and the other “beta.” The alpha subunits of hCG and TSH are identical but the beta subunits are different; but not by much. The beta subunits of hCG and TSH are about 40 percent identical. Given that mechanism, very high concentrations of hCG can actually stimulate the thyroid gland sending it a message to become hyperactive. In other words, hCG can sometimes act like TSH, which is the reason an elevated thyroid function test in the first trimester (which may be interpreted as an overactive thyroid), should be repeated in the second trimester when the levels of hCG have stabilized.

The fetal thyroid becomes active at 12 weeks. Until then, it depends on the mother’s supply for brain and nervous system development.

The thyroid also becomes slightly enlarged during pregnancy, but not enough to be viewed as abnormal during a physical exam. If a thyroid is noticeably enlarged, this could be a sign of thyroid disease. But again, thyroid problems are tricky to diagnose during pregnancy, because the thyroid hormones levels are higher than normal and also because fatigue and other symptoms of thyroid disease are often similar to normal pregnancy symptoms. That is why subspecialists, like myself, known as maternal-fetal medicine specialists, may need to be called upon by the midwife or obstetrician when caring for a pregnant woman with suspected thyroid dysfunction.

How Is Pregnancy Affected by Thyroid Problems?

Depending on whether you are dealing with hyper- or hypothyroidism, uncontrolled thyroid disorders in pregnancy can cause a number of health risks to mother and baby, including congestive heart failure, thyroid storm, neurologic deficits in the fetus, miscarriage, fetal growth restriction or neonatal Graves disease (hyperactive thyroid function in the newborn). Thyroid disease has an autoimmune component and other autoimmune diseases also need to be ruled out. Antibodies (TSI—thyroid stimulating immunoglobulins) from a hyperactive, hyperthyroid patient can cross the placenta and cause fetal goiter.

For these reasons it is extremely important that a woman with a thyroid disorder optimize her health by eating a balanced diet with iodized salt, taking a prenatal multivitamin that contains iodine (which the thyroid uses to make hormones), and taking the appropriate medication to control symptoms and regulate hormone production. Up to 10% of postpartum patients develop new-onset of Graves disease or autoimmune thyroid dysfunction. In my practice, patients who were six weeks postpartum all underwent thyroid function testing. It is not the standard, but, after diagnosing several patients with thyroid dysfunction, I did not want to miss an abnormally functioning thyroid gland in any of my postpartum patients. Thyroid conditions often require lifelong monitoring, but with the right medical attention, are easily controlled.